A hard metal composite body of hard material phases, such as tungsten carbide and/or carbides or nitrides of the elements of Groups IVa or Va of the periodic classification of elements, comprising reinforcing materials and a binder phase, such as cobalt, iron or nickel, is known from EP 0 448 572 B1 which contains as reinforcing materials either monocrystalline platelet-shaped reinforcements of borides, carbides, nitrides or carbonitrides of elements of the Groups IVa or VIa of the periodic classification of elements, or mixture thereof, or of SiC, Si.sub.3 N.sub.4, Si.sub.2 N.sub.2 O, Al.sub.2 O.sub.3, ZrO.sub.2, AlN and/or BN. The proportion of reinforcing materials amounts to 2 to 40% by volume, preferably 10 to 20% by volume.
U.S. Pat. No. 3,647,401 describes anisodimensional tungsten-carbide platelets with a maximum dimension between 0.1 and 50 .mu.m and a maximal expansion which is at least three times the minimal expansion. These platelets are bound by cobalt, in an amount of 1 to 30% in relation to the total body weight. The body has a density of 95% of the theoretical maximum density.
The CH 522 038 describes a hard metal sintered body with tungsten carbide particles, whose average grain size is smaller than 1 .mu.m, whereby at least 60% of the particles are smaller than 1 .mu.m. The metal phase proportion ranges between 1 and 30% and is composed of 8 to 33% by weight tungsten and 67 to 62% by weight cobalt. The anisodimensional WC particles should be aligned with their largest surface practically parallel to a reference line.
Finally the WO 96/22399 describes a multiphase sintered body, which has a first hard phase of carbides, nitrides, carbonitrides or carboxinitrides of the element of Groups IVa, Va or VIa metals of the classification of elements. The second phase consists of a solid solution with a grain size between 0.01 and 1 .mu.m of carbides, nitrides, carbonitrides and carbonitrides of at least two elements of the Groups IVa to VIa of the classification of elements. The binder is composed of cobalt, nickel, chrome, molybdenum and tungsten, as well as mixtures thereof. The sintered body can contain WC platelets of tungsten carbide with a size ranging between 0.1 and 0.4 .mu.m, which are formed in situ.
Since the first WC--Co hard metals have been invented and produced more than 70 years ago, activity in research and development laboratories has been directed to the improvement of the characteristics of these alloys and to optimize them for the ever increasing utilization possibilities. Particularly in the field of machining--a main utilization field of hard metals--during the further development of the materials to be processed, new hard metal alloys were continuously developed, which were characterized by an increase in not only the wear resistance of the cutting bodies, but also their strength. The coating of hard metal substrates with hard and wear resistant layers, as well as lately the introduction of refined and ultra-fine grained hard metals, in which the simultaneous increase of hardness and bending resistance was made possible with a decrease of the carbide size, represent important stages in the history of this development.
Particularly with the production of ultra-fine grain alloys of ultra-fine and nano-fine starting powders it had become clear that the conventional production methods reach limits during sintering, due to problems in the processing of powders and the grain enlargement.
This raises the problem whether and to what extent the conventional production methods have to be developed anew, or further developed, in order to promote continuing development of hard metal alloys, so that new concepts of composite cutting materials with improved characteristics can be implemented technically and economically. In this respect the sintering of hard metals in a microwave field offers itself as a new technology, affording entirely new solutions.
Microwaves are defined as an electromagnetic radiation in the frequency range of approximately 10.sup.8 to 10.sup.11 Hz (corresponding to the wavelength in vacuum of about 1 mm to 1 m). Commercially available microwave generators produce a monochromatic radiation, i.e. waves with a certain frequency. Widely used are generators with 2.45 10.sup.9 Hz, which corresponds to a wavelength of 12 cm. By contrast therewith the thermal radiation (Planck radiation) has a very broad frequency band width and in typical sintering processes it has its energy maximum at a wavelength of 1 to 2 .mu.m. Matter exposed to an electromagnetic radiation can become heated as a result of the interaction with the field, thereby draining the wave field of energy. Since this interaction is strongly frequency-dependent, the heating of matter takes place in the microwave field and also through thermal radiation based on various heating mechanisms.
Most solid materials have sufficiently strong absorption bands in the infrared wave length range and can be heated by heat radiation which is absorbed at the body surface. As a rule the transport of the heat energy towards the body interior takes place by heat conduction, resulting in a temperature gradient in the body from the inside out. If in a sintering oven there is a batch of parts (sinter charge), which is heated by a peripheral heat conductor, then for reasons which are analogous to the case of the individual body, a temperature gradient develops across the sinter charge. If the aim is to insure a certain temperature homogeneity inside the sinter charge, i.e. to keep the temperature gradient small, then the heating rate has an upper limit because of the thermal inertia of the charge and the oven. Therefore a certain minimal dwelling time is predetermined for corresponding temperatures.
The interaction of matter with a microwave field takes place through the electric dipoles existing in the material or free charges. The scale of the absorption characteristics of materials for microwaves extends from transparent (oxide ceramic, several organic polymers), through the partially transparent (oxide ceramic, nonoxide ceramic filled polymers, semiconductors) up to reflective (metals). Further the behavior of a material in the microwave field depends on the microwave frequency and in large measure upon the temperature. A material which at room temperature is microwave transparent, can at higher temperatures become strongly absorptive or reflective. For most material the penetration depth of the microwaves is considerably greater than for the infrared radiation, which depending on the sample size, results in the fact that the material--in contrast to the "skin heating" of the infrared radiation--can be heated through its volume with microwaves. The penetration depth of microwaves of the frequency 2.45 GHz at a temperature of 20.degree. C. (calculated from measuring the dielectric constants) varies in different materials and has the following values: 1.7 .mu.m for aluminum, 2,5 .mu.m for cobalt (as an example of a metal), 4.7 .mu.m for WC and 8.2 .mu.m for TiC (as examples of massive semiconductors), 10 m for Al.sub.2 O.sub.3 and 1.3 cm for H.sub.2 O (as examples of insulators) and 7.5 cm for WC with 6 M % Co, 31 cm for Al.sub.2 O.sub.3 with 10 M % Al and 36 cm for Al.sub.2 O.sub.3 with 30 M % TiC (as examples of powder metal green compacts).
The sintering of ceramic materials, such as silicon nitride, aluminum oxide or a mixed ceramic in the microwave field has been known for more than 10 years. But since the beginning of worldwide activity in the field of microwave sintering, it was prevailing opinion that this technology can not be used for the sintering of materials with a high electric conductivity, such as for instance hard metals. This opinion was based on the fact that massive metallic bodies can practically not be heated, since they reflect the microwaves well due to their high electric conductivity and only a superficial layer several micrometers thick can be heated via eddy currents. However it has been surprisingly found that the dissipation behavior of metallic-ceramic compressed bodies produced according to powder metallurgy depends not only on the electric conductivity of the participating phases, but in large measure on the microstructure, and that an effective heating of metallic powders is very well possible. In a sufficiently fine distribution of the metallic phases in a mixture with nonconductive or semiconductive powders (such as for example WC--Co compressed powder bodies) an extremely effective heating takes place, which seen microscopically is based on "ohmic losses" between the grains and high frequency eddy currents at the individual grain. From the previously mentioned penetration depths the behavioral difference in the microwave field between massive bodies and compressed bodies produced through powder metallurgy can be clearly seen. More precise tests have shown that the penetration depth of the microwaves in metallic, respectively semiconductive compressed bodies also depends on the power of the microwave field and decreases clearly at higher output densities. This phenomenon is explained by the shielding of the sample with electrically conductive plasmas, which in the marginal area of the porous compressed bodies are ionized in the pores after the penetrating power has been reached.
By taking into consideration the interaction of the of microwaves with the introduced green compacts produced through powder metallurgy, the hard metals can be sintered by means of microwave until they reach their final theoretical density.